In computing, an air gap is often the reason things don’t work. For example, the gap between the network cable and the network socket which stops your network working. Plug it in, close the gap and hopefully data will be flowing again. Of course, sometimes you don’t want the data to flow too easily. Creating an “airgapped” computer or network, with no connection to external networks, is a security technique that comes into play in some circumstances.
I was interested to read an article about research into ‘Fansmitter’ malware that proposes one way to bridge such a gap by hijacking the system fan and using subtle changes in pitch as a vehicle for transmitting data. Mind you, if you check the abstract for the paper that report is based on, you’ll see that it only runs a a maximum of 900 bits/hour. That is pretty slow. If you had a text document using ASCII character encoding (1 byte or 8 bits per character), that would be not much over 100 letters an hour – if this post was the payload, that wouldn’t get you to the end of the second sentence in the first hour and you probably wouldn’t be this far by tomorrow.
All in all, probably not a particular worry, especially given the other challenges (like getting the malware onto the airgapped computer and setting up a listening device in range; if you can get that close, you might as well just steal the machine!). It is an interesting concept though and, while just nibbling away a little of the protective wall, each bit potentially contributes to a more significant future breach of the defences.